Because when it’s NSFW it’s probably an image or video which uses more bandwidth than text does, so if you’re scrolling all those images and previews need to be downloaded and if it’s prefetching more than just a thumbnail while you scroll you can end up with a lot of data transfer.

That or they’re running an instance and want people to leave here and join theirs. Would make sense for someone who wants to manipulate content and people, crush the popular instance and you’re more likely to inherit those users.